Singlet-triplet perturbations in formaldehyde

CHEMICAL PHYSICS LET??ERS

Volume 53. number 1

SINGLET-TRIPLET

PERTURBATIONS

1 JarmarYX978

IN FORMA.LDEHYDE *

and D.A. RAhISAY and SM. TILL** Henberg histiture of Astrophysics, Natioltd Research 6%unci~ of &nada. Ottawa. Ontario, Canada KIA OR6

Received 12 September 1977

ihe magneticrotationspectrumof formaidefiydein the regionof 3260 A has been studiedunderhigh resohxtionusing magnetic!flux densitiesrangingfrom 80-5000 G. Approximately40 magneticallysensitiverotationallevelshavebeen identified. hfatrixelementsand possiblemechanismsare discussed. Singlet-triplet perturbations in the near ultraviofet spectrum of formaldehyde have been known since the cfassic studies of the magnetic rotation spectrum of the 3260 A band by Kusch and Loomis [I ] . Detailed investigations of these perturbations have been carried out by Brand and his cofieagues [2-4 who postulated that most of the perturbations could be explained on

the basis of zmintersystem spin-rotation coupling between the 2241 level of the ‘AZ state and the I f2241 level of the 3Az state, Three more perturbations were attributed to a vibronic spin-orbit coupling of the singIet level with the 1 I22 level of the 3A2 state. The work of Brand et aI. was based primarily on

Zeeman studies and on local perturbations in the rotational energies of the 2241 level of the singlet state. We have re-investigated the magnetic rotation spectrum @IRS) under high resofution using an apparatus described earlier [S] and a 7.3 m Ebert spectrograph. The pressure of formaldehyde was i-10 torr, the path length 1 m, and magnetic fIux densities up to 5000 G were-used. The magnetic rotation spectra in the region of the 3260 a band were ~articuIarIy rich in I&es

and good spectra were obtained with flux densities as * NRCC No. 16340. ** NRCC Research Associate14

low as SOG and exposure times of l-2 hours. Under these conditions any Zeeman shifts of the magnetic rotation lines are
3~690,4i6b) 3~&s3,23 xbl 30689.935 30852.75~

MRS term

MRS term

b) h)

30684,~76b)

3~@s~.lo~b~

30616.33~ 31050.034

30631,579 308S2~760

30631,673 308S2,854 3061~693

30604,226 31~5~.11~

30604.159 3 i~S~.O43

3~618.615 3~852,749

306I9.137 3~853.27i

30852,827

3~559.238 31050.044

3~853.284

3~SS8.3~ 308S3.292

3105~,~3? 31~50.099

3~SI4‘105 310S0.041 30514s 12 3lUSO,lO8

30557.781 30852,751

3~852~%33

3O~S7.861 30852.831

31~38,048

31~38.428

31~3%‘455

Mean

@ Ov~rIapp~d*

a) The MRSlineswereobtainedusinga flux~cnsi~~of 1606. The upper state termvnlucswerederivedusingthe ~rouodstate termvfues from ref. [8J+

MRY term

ats ferm

(c) 182&jupper level

3~695,~l~ 30852.830

ab term 30649,539 30852,824

3lQ38.047

(b) 122,toupper ICVCI

304g9.198

30651.994

31038,062

307fldi21 31038‘~36

31038,444

MRS term

3~4g9.59~

p'r

30489.59& 31~38.447

It) b)

PQ

30717,995 31~38,41~

pR

MRS term

‘P

3~718,04? 31~38,462

3o686.386 bl 31~3%.413b)

fQ

abs term

(a] 138upper level

CR

Table1 RotatIonalassl~nm~nts and upper state termvaluesfor some iinos inthe 3260 A band of formaldch~d~ and associatedMRSlines(in cm-t)a)

Volume 53, number 1

with the 12,,,, and 12,,11 excited state singlet levels and for these levels (and also the I24 g level) more than one set of tripler terms is found’in addition to the singlet term. The strongest MRS lines associated with the 12?,19 level are given in table lb. For some excited state singlet levels, e.g., 7,,, , 1 82,16, the only MRS lines lie almost coincident with the absorption lines. In these cases we assume that the singlet and triplet levels are very nearly degenerate. The data for the 18t,16 level are given in table lc. The intensities of the magnetic rotation (and MCD) lines are derived prirnaiiy from the electric dipole matrix elements of the singlet-singlet transitions. Contributions from triplet-singlet intensity matrix elements can be neglected since the intensities of the MRS lines are about two orders of magnitude stronger than those obtained in the MRS studies of the 0+--O and 1*-O bands of the 3A2 - ‘A, system 191. Calculations reveal that ‘R and PP lines should be the strongest, followed by ‘P and PR lines; *Q and *Q lines should be somewhat weaker. These predictions are in general agreement with experiment, although effects due to self-absorption and absorption by overlapping lines have been noted. Altogether 24 magnetic levels have been identified from MRS spectra obtained with a %JX density of 160 G (table 2). These levels are classified according to the singlet levels with which they are associated. The shifts of the singlet levels caused by local perturbations are also given and were obtained from a least squares fit of the rotational assignments of the 3260 A band [S] _ It should be noted that half of the magnetic levels have not been previously reported; many are associated with singlet levels which are net appreciably displaced (
1 Jlinualy 1978

CHEMICAL PHYSICS LETTERS

Table 2 h¶agneticaUysensitive levels associated with the 3260 A band of formaldehydea) Singlet level

Displacementc)

Singlet

Dispkementc)

(cm-‘)

level

(cm-t)

:$p’

+0.038 -0.03 1 -0.029 +0.010 +0.019 +0.078 -0.018 -0.005

-0.03 d)

7097 llo,11b) 180,113 2OOJO ll*,gb) 1~2,lO b) 122,I 1 b, 1% 14

+0.230 -0.02s d) -0.02d) +0.06 1 -0.06 I

15&b) 174,13 174

13,Q

-0.200

182316

14

165

+0.003

126

+o.ooo

156

+CLO45

166 b)

-0.016 +0.047 -0.06 d)

:;xobi,

-0.045 -0.110

133:llb)

-0.074

1% 138

a) identified using magnetic rotation spectra at 160 G. All these levels have also been identified in the MCD experiments. The following additional levels were observed with higher fields: 1~0,12W 71,7> 10*,9(?),111,*ob)(?). 121,J-J). 121,a. 7 2,6*

112,10

194.15,

b),

204,~~

113,9b).

173,15c%.

204,17<%

66.76,

134,9b). 176,

177,

134,lOW 1%

159(?h

1210.

Denotes magnetically sensitive level reported by Brand and colleagues [ 3,4] _ C) From the rotational analysis of the singlet-singlet band [S] . d) The displacements quoted for 70 ?, 18o,ra, 200,~~ and 13a are the additional local perturbaiions superimposed on larger and more extensive perturbations. b)

lr e =3re, AJ=o,

Q?,.=3r

ev*

5K=O,+2,5N=O,~I

(ii) vibronic spin-orbit

; mechanism

Irevx 3r.__.3R,;AJ=O,M=O,5N=O,+1; ‘revx 3rev IR,

orR.,,;~=O,AfC=-+i,&V=O,i1.

The observation of a large number of magnetically sensitive rotational levels strongly suggests that most of these levels interact via one or more AK 5 0 mechanisms since a AK = i- 1 mechankm can only account for a few levels. In an attempt to determine the piincipal mechanisms involved, all possible vibronic origins for the magnetically sensitive triplet levels were calculated for the selectionruies~=O,~=O,+land~=O,t-1,~2. The term value for each level was deperturbed using the shift of the corresponding singlet level, and the ap-

CHEMICAL

Volume 53, number 1

Table 3 PossibIe band or&us in the regton of 30650 cm-r vibronic triplet statea) Levelb)

113,8

13s 124.9

1 lo,1 1 122,ll

122,lO I56

133,ll 133,lO

1 January 1978

PHYSICS LETTERS

for a B2

Calculated band origin (cm-t)

30651.9 30650.9 30650.5 30649.8

30649.8 30649-7 30649.6 30649.2 30649.1

a) There &e no further values within 1.5 cm-l at either end of the table. The f&t two levels in the table were assignedby Brand and Stevens [3J to a different mechanism. b) Note that the triplet level is designated by the singIet level with which it interacts.

propriate rotational term value, calculated from the constants of Brand and Liu [4], was subtracted. The origins were sorted into four groups depending on whether the vibronic symmetry of the triplet state is A,, A2, B, or B2. Each group comprises 70-250 values, spanning ranges of 100-500 cm-l. It was found that only a small fraction of the values he close to any single origin and, furthermore, that this fraction could not be significantly increased by adjustment of the rotational constants. As an example of the difficulties encountered in identifying mechanisms, we show in table 3 all of the values in the region 30650 * 2 cm-l for a B2 vibronic triplet state, one of the origins given by Brand and Stevens [3] _We note that it is not possible to obtain a single set of rotational constants which will give a consistent value for vo. This latter point is implicit in the work of Brand and Stevens who had to adjust the values of B and C within specified

limits in order to explain the individual perturbations. ft appears that more than two triplet levels are needed to explain all the observed magnetic rotation lines. Furthermore, it is probable that the triplet levels are themselves perturbed, e.g., by Coriolis or Fermi interactions within the triplet manifold. This latter point is not surprising since the triplet levels he =5450 cm-l above the origin of the triplet system. Under the circumstances it may not prove possible to give detailed molecular constants for the triplet levels. Nevertheless, the identification of singlet-triplet perturbations is important, for example, in studies of radiative lifetimes [lo], and magnetic rotation studies provide a sensitive method for revealing such perturbations. We wish to acknowledge stimulating discussions with Drs. J.C.D. Brand and J.L. Hardwick.

References [ L] P. Kusch and F-W_ Loomis, Phys. Rev. 55 (1939)

850. [2] C-G. Stevens and J.C.D. Brand, J. Chem. Phys_ 58 (1973) 3324_ 131 J-CD. Brand and C.G. Stevens, J. Chem. Phys. 58 (L973)

3331. [4] J.C.D. Brand and D-S. Liu, J. Phys. Chem. 78 (1974)

2270. [S] W_ Goetz, AJ. McHugh and D.A. Ramsay, Can. J. Phys. 48 (1970) 1. [61 G.A. Osborne, I. MoI. Spectry. 49 (1974)

48.

[7] J-E. Pa&m, H-G. Poole and W-T. Raynes, Proc. Chem. Sot. (1962) 248; J-E. Parkin, thesis, University of London (1962).

[81 F.W. Birss, D.A. R~rnwy and S-M_Tii, unpublished. [9] J-M. Brown. A-D_ Buckin&am and D.A. Ramsay, Can. J.

Phys. 54 (1976) 895. [ 101 K-Y. Tang, P-WV.Fairchild and E-K-C. Lee, J. Chem. Phys. 66 (1977)

3303.

17